1. Field of the Invention
The invention relates to a digital circuit for sampling rate variation and for signal filtering, including an input, an output, means for varying the sampling rate and for generating a phase change in a digital system, and a lattice wave digital filter having a plurality of filter branches, especially two, which are connected to the input and are connected through an adder to the output, the filter branches having filter subgroups with basic filter elements each formed of one two port adaptor made up of adders and multipliers and one time-lag device.
2. Description of the Related Art
Digital systems often operate at different sampling rates, in order to reduce the number of operations to be performed per second. Sampling rate variations, that is sampling rate increases or decreases, are necessary in order to connect parts of the system operating at different sampling rates. Each variation in the sampling rate can be broken down into two steps, namely a compression or expansion of the sampling rate and decimizing or interpolating digital filtering, which approximates an ideal low-pass or bandpass damping characteristic.
In larger and in particular in multi-rate digital systems, damping specifications are often facilitated by means of preceding filtering steps. Correspondingly, the sampling rate compressor or expander can be combined with a digital filter, which has different damping specifications from purely decimizing or interpolating digital filters. In any case, the problem is to find effective possibilities for combining the two steps. In so doing, it is always desirable for the digital filter to be driven at the lowest possible sampling rates, in order to keep the expenditure for circuitry, components and programs as low as possible.
In the class of recursive digital filters, an article by A. Fettweis and J.A. Nossek entitled "Sampling rate increase and decrease in wave digital filters", in IEEE Trans. Circuits Syst., Vol. CAS-209, No. 12, 1982, pp. 797-806, describes the use of a wave digital filter having a ladder structure in combination with a variation of the sampling rate. Wave digital filters correspond to LC and microwave filters in analog filters and have the same stability. They also have very good sensitivity, or high tolerance. The apparatus described above has some advantages, in particular those of a low power loss for a given filter structure, a wide dynamic range and a low bandpass sensitivity for the filter coefficients. However, the entire filter always operates at the higher sampling rate.
It is known from an article by A. Fettweis entitled "Transmultiplexers with either analog conversion circuits, wave digital filters or sc filters - A review". in IEEE Trans. Communications, Vol. COM-30, No. 7, 1982, pp. 1575-1586, that wave digital filters having a grid or lattice form and bireciprocal characteristic function can operate at half the otherwise required sampling rate. It is also known from this source to link pseudo-multiple-path digital filters, base digital filters and means for varying the sampling rate to one another. In order to permit an understanding of the present invention, the concept on which the bireciprocal digital filters are based will be briefly explained below with the aid of FIGS. 1 and 2, in terms of the known filter structure, beginning with the principles of its construction.